1. Field of the Invention
The invention relates to well production control systems, and more particularly, to a computer controlled gas lift system.
2. Prior Art
In the operation of hydrocarbon production wells, gas lift apparati are occasionally employed to stimulate movement of fluid uphole. The operation ranges from simply pumping high pressure gas downhole to force fluids uphole to pumping additional fluids into the production fluid lowering the specific gravity thereof and thus increasing the "interest" of the fluid in migrating toward the surface. Gas lift apparati are also periodically employed when, a mixture of oil and water collects in the bottom of a gas well casing and tubing in the region of the producing formation and obstructs the flow of gases to the surface. In a "gas lift" well completion, high pressure gas from an external source is injected into the well in order to lift the borehole fluids collected in the well tubing to the surface to "clear" the well and allow the free flow of production fluids to the surface. This injection of gas into the well requires the operation of a valve controlling that injection gas flow known as a gas lift valve. Gas lift valves are conventionally normally closed restricting the flow of injection gas from the casing into the tubing and are opened to allow the flow of injection gas in response to either a preselected pressure condition or control from the surface. Generally such surface controlled valves are hydraulically operated. By controlling the flow of a hydraulic fluid from the surface, a poppet valve is actuated to control the flow of fluid into the gas lift valve. The valve is moved from a closed to an open position for as long as necessary to effect the flow of the lift gas. Such valves are also position instable. That is, upon interruption of the hydraulic control pressure, the gas lift valve returns to its normally closed configuration.
A difficulty inherent in the use of single gas lift valves which are either full open or closed is that gas lift production completions are a closed fluid system which are highly elastic in nature due to the compressibility of the fluids and the frequently great depth of the wells.
Prior art flow control valves for downhole applications, such as single gas lift valves per area, include the disadvantage of not providing a substantial amount of control over the exact amount of gas entering the well. This is because the valve is either open or closed and cannot be regulated. Hydraulically actuated downhole flow control valves also include certain inherent disadvantages as a result of their long hydraulic control lines which result in a delay in the application of control signals to a downhole device. In addition, the use of hydraulic fluids to control valves will not allow transmission of telemetry data from downhole monitors to controls at the surface.
Boyle et al patented a system capable of adjusting the orifice size of the valve through a range of values, thus providing a broader control over the amount of gas being injected into the system. U.S. Pat. No. 5,172,717 to Boyle et al discloses a variable orifice valve for gas lift systems. The system allows for adjustment of the flow through a particular valve body thereby allowing tailoring of the flow rate and alleviation of some of the previous problems in the art. The variable orifice valve allows greater control over the quantity and rate of injection of fluids into the well. In particular, more precise control over the flow of injection gas into a dual lift gas lift well completion allows continuous control of the injection pressure into both strings of tubing from a common annulus. This permits control of production pressures and flow rates within the well and results in more efficient production from the well.
The '717 patent solved many of the aforementioned problems with its variable orifice valve. Variable opening however provides some of its own inherent drawbacks such as lack of reliability of "openness" over time. More particularly, scale and other debris can build up and prevent movement more easily on orifice closures which are responsive to small increment movements and, in general, are only moved or adjusted by such small increments. Thus when conditions change downhole over time the variable orifice valve may be unable to comply with the changing conditions and would need to be replaced.
Another adjustable gas lift valve is disclosed in U.S. Pat. No. 5,483,988. The disclosure teaches a system having several parts or features but particularly includes an adjustable flow gas lift valve which includes a flow port and a plurality of differently sized nozzles selectively alignable with the port. Sensory devices are employed to maintain information about the state of the valve assembly. The variable nozzles are located on the actuator and, therefore, can be rotated into alignment with the orifice port to regulate the amount of gas flowing therethrough as desired.
Fully open/fully closed valves provide a large relative movement and tend to jar loose any buildup so that valve serviceability is maintained for a longer period of time. Therefore, these valves have a significant service life advantage over the more "advanced" variable opening valves. Also, where a plurality of these valves are employed in a given area, the closing of some (or opening) does not subject the individual valves to the same torsional forces because all flow is not pitted against a single structure. Thus opening or closing of the valves does not lead to excessive wear of valve components. The industry is in need of a system that experiences the benefit of variable orifice valves while concurrently benefitting from the serviceability of fully open/fully closed valves.